1. Field of the Invention
The present invention relates to a transmission system for a bicycle. It also relates to a device for controlling the transmission system, as well as to a bicycle equipped with the transmission system and control device.
2. Description of Background and Relevant Information
Usually, a bicycle is equipped with a transmission system including a transmission chain, a rear sprocket cassette, a set of two or three front chain wheels, and rear and front derailleurs to transfer the chain from one sprocket to another, from one chain wheel to another, respectively. Currently, the sprocket cassettes include up to ten sprockets.
For such a system, the range of transmission ratios between the bottom bracket assembly rotating speed and the wheel rotating speed extends between two extreme ratios, one of which corresponds to the largest chain wheel and the smallest sprocket, and the other corresponds to the smallest chain wheel and the largest sprocket.
There is a certain number of intermediate ratios between these two ratios, which depend on the number of sprockets and chain wheels.
During a ratio change control, the variation in the transmission ratio does not have the same amplitude, depending upon whether one changes a chain wheel or a sprocket. It is known that for the various chain wheels, the ratio ranges obtained by sweeping the entire sprocket cassette overlap. Such that out of the set of possible ratios, which is equal to the number of sprockets multiplied by the number of chain wheels, only a portion, approximately one half, is actually exploited, or actually useful. There are overlapping zones in the ratio ranges, i.e., the same ratio, or almost the same ratio can be obtained with two different combinations between the chain wheels and the sprockets. If a progressive variation in the transmission ratio is desired, a change of chain wheel must be accompanied by a change of one or two, or even three sprockets in the reverse direction to compensate for the jump due to the change of chain wheel.
Such a maneuver is relatively slow and causes a break in the pedaling rate. This break is all the more pronounced as the change of chain wheel requires momentarily lightening the force exerted on the pedals to reduce the tension of the tensioned chain strand.
In addition, such a system lacks uniformity in the progression of the ratios, i.e., the rate of increase or decrease in the transmission ratio varies in an irregular manner when shifting gears, mainly upon a change of chain wheel.
This rate is related to the variation in torque which the cyclist exerts on the pedals. Certain gear shifts yield substantial variations in torque or, conversely, small variations resulting in a lack of ease of use for the cyclist.
Centralized control devices are also known, which synchronize the front and rear derailleurs so as to ensure a progressive variation in ratios. In particular, these devices are adapted to simultaneously control a change of chain wheel or a change of sprocket. A device of this type is described, for example, in the published patent document FR 2 750 669. The advantage is that the gear shifts are guided by a single control, even during a change of chain wheels. However, the problems of progressive variation in ratios and the shifting of the tensioned chain strand on the front are not resolved. Moreover, in the overlapping zones, this system imposes the combination of the chain wheel and sprocket; it does not leave any choice. Furthermore, it imposes a change of a plurality of sprockets simultaneously to a change of chain wheel. Therefore, the system is not very flexible.
For these conventional systems, there is also a problem related to the chain alignment, i.e., it is not recommended to associate the small sprocket with the small chain wheel, and conversely. An incorrect alignment causes a loss in efficiency, undesirable friction and noise during a change of sprockets and chain wheels. Certain combinations of sprockets and chain wheels are not used for this reason.
To reduce the number of ratios and avoid overlaps, it has been proposed to guide a change of transmission ratio solely by a change of sprockets.
Thus, the published patent document EP 849 155 proposes a transmission system including fourteen rear sprockets and a front chain wheel. To resolve a space requirement problem, the sprockets are very close to one another, and the chain has a particular construction that is narrower than a usual chain. In particular, the chain proposed in this patent document has inner links bent in the form of a bridge. Such a chain is not as strong as a more conventional chain, because the inner links are capable of bending.
Another cassette of fourteen sprockets is described in the published patent document EP 561 380. The sprockets here are juxtaposed, and the chain is meshed on one or the other by its lateral small plates.
A first disadvantage is that these systems require sprockets and a chain that have a very specific construction.
Another disadvantage is related to the chain alignment. Indeed, the sweep of the sprocket cassette requires that the chain be substantially out-of-line relative to the nominal angle given to it by the front chain wheel, resulting in a rapid wear and tear, loss in efficiency, and noise due to friction.
An object of the invention is to provide an improved transmission system that ensures a more varied progressiveness for the transmission ratio.
Another object is to provide a transmission system having two sources of variation in the transmission ratio, as are the sprockets and the chain wheels, where for the same ratio range, the number of combinations is further reduced, and the provision of chain wheels and/or sprockets or equivalent is less substantial.
Other objects and advantages of the invention will become apparent from the description that follows.
The transmission system of the invention is provided for a bicycle having a frame, a front wheel, a rear wheel with a hub and a hub shaft, and a drive bottom bracket assembly with a crank axle, including a transmission chain, a primary transmission sub-assembly having a set of sprockets provided to be mounted on the rear wheel, whose number of teeth varies generally, for at least a portion of them, along a geometric progression with a ratio of the numbers of teeth of two adjacent sprockets having a mean value n, such as n greater than 1, with a rear derailleur provided for transferring the chain from a given sprocket to an adjacent sprocket. At least one crown is provided for driving the chain associated with the bottom bracket assembly, and a secondary transmission sub-assembly is provided having at least two drive output ratios, whose ratio of the highest drive speed to the lowest drive speed is equal to a value R comprised between 1 and n.
According to a general feature of the invention, at least one intermediate ratio is created, with the secondary transmission sub-assembly, between the transmission ratios corresponding to two successive sprockets. In other words, at normal speed, the secondary transmission sub-assembly is activated between two changes of successive sprockets to change the transmission ratio and create an intermediate ratio. It is activated in the inverse direction at the time of the next change of sprocket.
The chain wheels of a conventional system yield a substantial variation in the transmission ratio, always greater than that induced by the sprockets. Instead of this, the secondary sub-assembly here yields a small variation in the transmission ratio, smaller than that induced by the sprockets.
With the system of the invention, one obtains a ratio range that is equivalent, or even wider than with a conventional system with a reduced number of sprockets, and a variation in the ratios that is better controlled, i.e., more progressive and more uniform.
The simultaneous change of sprocket and of ratios of the secondary sub-assembly here poses less problems than a traditional change of chain wheel. Indeed, as will become apparent in the following description, the secondary sub-assembly preferably does not act on the tensioned chain strand. In addition, as the ratio of the output ratios is comprised between 1 and the value R close to 1, the variation in the transmission ratio which it generates is small, in any event markedly smaller than that which occurs during a traditional change of chain wheel.
Furthermore, as the change of ratios of the secondary sub-assembly occurs upon each change of sprocket and between two changes of successive sprockets, it is easier to design a centralized control that guides the two transmission sub-assemblies.